High capacity and high rate rechargeable batteries with low cost and improved safety characteristics constitute a major requirement for electric vehicles, portable electronics, and other energy storage applications. Year-to-year electrochemical performance improvements in lithium-ion batteries (LIBs) are typically limited to 3-4%, with a major bottleneck being the lack of appropriate materials to satisfy the energy and power density requirements. Progress in nanostructured anodes has improved the potential of the practically achievable capacity and rates. For example, high capacity anodes such as silicon, which have been studied since the 1980s, have been found to overcome structural degradation problems through the use of nanowire morphologies. However, batteries utilizing silicon anodes can still only achieve a 30% gain in energy density due to the low capacity of the cathode: current cathodes have practical capacities of 150-180 mAh/g.
Both diamond cubic and amorphous silicon can reversibly alloy with Li electrochemically, making silicon a promising high energy density anode for Li-ion batteries. The theoretical charge storage capacity for silicon is about 4000 mAh/g, more than an order of magnitude higher than for graphite, the existing Li-ion battery anode. However, the structural changes that occur during this process results in the formation of a great deal of stresses than can lead to pulverization of the silicon. This is attributed to a 300% change in volume between the unlithiated and lithiated phases. The use of nanostructuring has been applied to allow the silicon to undergo this volume change without fracturing or pulverizing. This concept has been demonstrated with various silicon nanostructures. However, this strategy relies on the use of engineered space within or in between the nanostructured silicon, which effectively gives it space to expand and contract without impinging on itself. Agglomeration and degradation of the engineered structure after many lithiation/delithiation cycles may lower the effectiveness of this strategy. The nanostructuring also does not address the volume expansion of silicon upon lithiation.